To stop the spread of Communism after World War II, the United States established a policy known as "containment" which sought to halt communism's rise.
The policy revolved around the idea that they would contain communism to countries where it already was and prevent it from spreading to other countries. They feared it would be like dominoes where one country's fall would lead to others and so until everything is communist. That's why they intervened in places like Vietnam and Korea.
